Fractionation of phosphate oxygen isotope in the leachate of phosphogypsum during its infiltration.

Abstract

The oxygen isotope composition of phosphate (δ¹⁸O-PO₄^3-) is a valuable tool for tracing the origins of phosphate (PO₄^3-) in the environment. However, the presence of microorganisms can pose challenges in identifying potential sources of PO₄^3- in complex environment, such as groundwater. Given that phosphogypsum leachate (PGL) significantly contributes to groundwater P pollution, the infiltration of PGL into aquifers may alter the fractionation of δ¹⁸O-PO₄^3-, thereby impacting the application of δ¹⁸O-PO₄^3- techniques in tracing the origin of PO₄^3- within groundwater. The present study investigated the influence of environmental conditions and the properties of PGL, such as low pH and high concentrations of fluoride and sulfate, on the fractionation of δ¹⁸O-PO₄^3-. The results revealed that that the presence of microorganisms led to an increase in the δ¹⁸O-PO₄^3- from 10.13 ‰ to 10.92 ‰, while no significant change was observed in a sterilized environment. The fractionation of δ¹⁸O-PO₄^3- adhered to the Rayleigh fractionation model with a fractionation coefficient of -2.97 ‰ (R² = 0.977). Furthermore, high concentrations of fluoride ions were observed to impede the kinetic fractionation of δ¹⁸O-PO₄^3- by suppressing microbial activities, which was consistent with the Rayleigh fractionation model. Additionally, low pH inhibited pyrophosphatase activity, thereby impeding equilibrium fractionation of δ¹⁸O-PO₄^3-. Under anaerobic conditions, sulfate ions contributed to transient fractionations through sulfate reduction-mediated Fe/Al-P release, however, this effect disappeared upon reaching adsorption equilibrium. Overall, this study provides valuable insights into understanding the factors influencing δ¹⁸O-PO₄^3- fractionations during the infiltration of PGL from soil to groundwater.